1. Field of the Invention
The present invention relates to a motor controller and a method for controlling a stepping motor.
2. Description of the Related Art
A method for driving an inductive load is disclosed in JP-A-H10(1998)-080194, which combines regenerative electric current and flowing electric current to cause switching current flowing in an inductive load to be appropriate.
In paragraph 0023 of JP-A-H10-080194, it is described that “an H-bridge circuit is configured to allow current to flow in an inductive load in opposite directions, that is, in forward and reverse directions, with four semiconductor switching elements and fly-wheel diodes anti-parallel connected to the semiconductor switching elements, respectively, so that, when a current supply operation is performed to supply current from a power source to the inductive load, two of the semiconductor switching elements take a conduction state to cause current to flow in the inductive load in a desired direction, and thereby the current flowing in the inductive load is increased by such a current supply operation, with the result that, if current is equal to or greater than a predetermined reference current value, energy accumulated in the inductive load is released to control the current flowing in the inductive load.”
In paragraph 0024, it is described that “then, a control may be performed by two kinds of operations: one of them is performed such that one of the semiconductor switching elements takes the conduction state to form a closed current path with the semiconductor switching element that is in the conduction state and one fly-wheel diode, by the energy accumulated in the inductive load and thereby cause current to flow in the closed current path, and the other is performed such that all of the four semiconductor switching elements are interrupted to cause current to flow in the two fly-wheel diodes by the energy accumulated in the inductive load and thereby charge power.”
The former operation of causing the current to flow in the closed current path is referred to as a communication control.
Further, in paragraph 0026 of JP-A-H10(1998)-080194, it is described that “a predetermined frequency of driving cycle is made and a current supply operation is started by initiating the driving cycle, so that, if current flowing in the inductive load is equal to or greater than a predetermined value during a current supply operation, the current supply operation is terminated and energy accumulated in the inductive load is released, and, assuming that a predetermined period after the driving cycle is started is defined as a power regeneration period and a period from the end of the power regeneration period to the end of the driving cycle is defined as a current period, a power regenerating operation is performed within the power regeneration period after the current supply operation is terminated, and the communication control is performed within the current period.”
A motor is rotated by driving a plurality of drive coils in opposite directions (bipolar) and changing a phase to apply current. If switching elements provided on opposite sides of the coils, namely, a motor supply voltage side (high side) and a ground side (low side) are simultaneously turned off when the motor is rotated, a high voltage of flyback pulse (kick back) is induced by energy reserved in the coils.
A coil current is maximized at the time of the phase change. If the flyback pulse is induced, the coil current is discharged to the ground by parasitic transistor effect due to a Complementary MOS (CMOS) manufacturing process of an Application Specific Integrated Circuit (ASIC), thus causing a power dissipation. Therefore, the ASIC generates heat, so that an operation is limited at high temperature.
In a period when the flyback pulse is induced, the high-side switching elements are temporarily turned on instead of turning all the switching elements off, to cause the current discharged to the ground to flow through the high-side switching elements and thereby flow back to a supply power source of the motor. In this way, it is possible to decrease a power dissipation.
However, since a period when the flyback pulse is induced varies depending on a drive voltage of the motor, a drive load of the motor and a rotating speed, it is difficult to appropriately control a flow time depending on a motor operation. Thus, a case where a time for a communication control is not optimum may occur, so that a power dissipation may undesirably occur.
For example, if the time for the communication control is shorter than a period when the flyback pulse is induced, current flowing in the switching elements flows through a fly-wheel diode connected in parallel thereto to be returned, thus causing a power dissipation. In contrast, if the time for the communication control is shorter than the period when the flyback pulse is induced, an inductive load is short-circuited to act as a brake against the rotation of the motor, so that it is further difficult to control the speed of the motor, and it is impossible to measure a back electromotive force voltage for detecting a stall of synchronism after the speed of the motor is controlled.
Further, it is possible to reduce the effect of heat generation by mounting a heat sink to an ASIC and increasing a size of a substrate and an amount of copper. However, this method is problematic in that the size of the substrate or a case as well as manufacturing cost is increased.
An external attaching diode is added to every output terminal of the motor, and thereby coil current flows back to the motor power source without being affected by the parasitic transistor effect, so that it is possible to decrease a power dissipation. However, this method is likewise problematic in that the size of the substrate as well as manufacturing cost is increased because of increases in mounting part point and mounting area.